About Online Ruler

What we do

Online Ruler is a free, browser-based measuring tool that renders a centimeter/millimeter scale and an inch scale at actual size on your screen. It exists for one reason: when you need to measure something small and the ruler is in the other room, your screen should be enough.

The tool works on any device with a JavaScript-enabled browser - desktop, laptop, tablet, or phone. There is no account, no sign-up, no download, and no cost. Your PPI calibration is stored only in your browser’s local storage and never leaves your device.

Why calibrate?

Browsers assume a nominal 96 pixels per inch, but real displays range from roughly 80 to over 400 PPI. Calibrating once - against a credit card, banknote, or A4 paper - tells the ruler how many pixels equal one real inch on your display, so every millimeter and inch is drawn to true scale.

Our principles

• Free. No paywalls, no trial limits.
• Private. Measurements and settings stay on your device.
• Honest. An on-screen ruler is as accurate as your display and calibration allow - we tell you exactly how to verify it.